The present invention relates to a method of producing high-grade plastic parts and to an injection-molded part that is correspondingly produced.
In the case of high-grade plastic parts, particularly as regards the interior fixtures and fittings of motor vehicles, a major role is played by properties which relate to the feel when such parts are lightly touched or the feel when these parts are firmly seized with one""s hands.
In this way, for example, it is already known that a molded core part produced by conventional injection molding and known as an xe2x80x9cinsertxe2x80x9d in the case of certain plastic parts, should be foam-coated with a second component. Gearshift levers for motor vehicle transmissions can for example be produced using such a process. During foam coating with the second component, a layer is formed on the inner surface of the mold cavity; this layer results in a relatively hard skin that lends the plastic part a pleasant dry sensation. The relatively hard skin merges inwardly into a soft foam structure which results in a pleasant touch when firmly taken hold of with one""s hands. The combination of dry sensation and soft touch produces a special feel which can at most be surpassed by the feel of a part trimmed with leather.
Plastic parts which are produced in this manner, i.e. based on the so-called xe2x80x9cintegral foam techniquexe2x80x9d, and fitted with an insert, nevertheless require a relatively complex procedure and more or less intricate refinishing. Because this process tends to form voids, the number of rejects is also relatively high.
It is also already known that inserts should be extrusion-coated with a single plastic component. Because of its process reliability, this manner of extrusion-coating the insert is well suited to automatic production and also permits relatively short cycle times. Furthermore, plastic parts that are produced by extrusion-coating the insert do not require any refinishing whatsoever.
All the same, there is the disadvantage that depending on the component used to extrusion-coat the insert, only that feel which is characteristic of this component can be produced. If a hard component is for example used for the purpose of extrusion coating, a pleasant dry sensation is indeed obtained, but an unpleasantly hard touch is also produced when the article is firmly seized with one""s hands. Yet if a soft component is used for extrusion coating, this does indeed result in a pleasant soft touch, but it also leads to an unpleasant rubber-like, sticky sensation. The special feel that can be achieved by foam-coating an insert in the aforementioned manner cannot, however, be obtained by extrusion-coating.
It is also known that plastic parts should be produced according to the so-called mono-sandwich process. In this injection-molding technique, two or more different melt components in a specific layering are fed into an injection cylinder and are then successively pressed by the injection cylinder into a mold cavity. The first component pressed into the mold cavity forms a skin on the mold cavity walls; the next component makes contact with this skin on the inside of the skin in order to form an intermediate layer.
The xe2x80x9cmono-sandwichxe2x80x9d technique was developed to enable high-grade plastic to be used as sparingly as possible and only at those locations at which the high-grade material is necessary for optical or other reasons. In this way, yogurt pots can for example be produced by using the mono-sandwich method; the inner and outer walls of such yogurt pots consist of high-grade plastic approved for receiving food, while the inner wall and core area comprises an airtight plastic material.
Plastic objects such as handles and the like can also be produced using the mono-sandwich method; to achieve the static properties necessary for their function, the core area of such objects consists of a hard plastic material, while their outer area comprises a soft plastic material.
The object of the present invention is to devise an innovative method with which the special feel that could previously be achieved only as a result of complicated foam-coating can also be produced by means of extrusion-coating an insert.
In accordance with the invention, this object is solved by an injection-molding process in which a layering of at least one hard component followed by at least one soft component of a plastic material is built up in the injection cylinder and then these components are successively fed in this order from the injection cylinder into the mold cavity for the purpose of extrusion-coating the insert.
The fact that this layering of hard and soft components is injected onto an insert, i.e. just one side of the insert is e.g. provided with the above order of layers of different components, should also be subsumed here by the term xe2x80x9cextrusion-coatingxe2x80x9d.
The invention is based on the idea that an insert is extrusion-coated using a mono-sandwich process, which was designed per se for completely different areas of application, such that a relatively hard skin is produced on the outside of the finished plastic part. At least one soft layer adjoins this hard skin in the direction of the insert. The aforementioned dry sensation with a soft touch, which could hitherto only be obtained by complicated foam-coating, is achieved as a result of the relatively hard outer skin on a relatively soft layer. The hard component also makes contact, in a thin layer, with the insert, thus achieving excellent adhesion with the insert, which is very tricky in the case of a soft component that produces the desired soft sensation.
The new method permits completely automatic production with short cycle times, and the plastic parts produced in this manner do not need to be at all refinished.
It is advantageous if what is used as a soft component is a plastic material which in its finished state exhibits a hardness of 7 to 40 Shore A, preferably 30 Shore A, and particularly 7-8 Shore A in order to achieve a very xe2x80x9csoftxe2x80x9d feel.
A plastic material which in the plastic part""s finished state exhibits a hardness of 70 to 100 Shore A, preferably 90 Shore A, is particularly suitable as a hard component. The method according to the invention is in principle suitable for producing every kind of plastic part. This method is particularly advantageous, however, for plastic parts which are needed for motor vehicle interior fixtures and fittings and which come into contact with one""s hands. Examples of such plastic parts are gearshift levers, handles, buttons, glove compartment lids and the like.